Ceramic pressure sensors comprise a platform and a measuring membrane, wherein the measuring membrane is joined with the platform by means of an active hard solder, or active braze. A suitable active hard solder, or active braze, for joining ceramic parts of corundum is, for example, a Zr—Ni—Ti alloy, since such alloys have coefficients of thermal expansion compatible with corundum.
European Patent EP 0 490 807 B1 discloses such an active hard solder, or active braze. European Patent EP 0 558 874 B1 discloses a method for manufacturing rings of such an active hard solder, or braze. Rings of active hard solder, or active braze, for connecting the measuring membrane and platform are placed as space holders between the two parts, and melted in a high vacuum, soldering process, whereby a pressure tight and high strength annular connection arises between the two ceramic parts. An alternative for applying the solder in the form of prefabricated rings is to provide the active hard solder, or active braze, in a screen printing process. A screen printable paste of the active hard solder, or active braze, and a method for its manufacture is disclosed in Offenlegungsschrift (laid open application) EP 0 988 919 A1.
The rings can, however, only be manufactured with reproducible quality with a minimum thickness of, for instance, 30 μm and also the screen printable paste includes granulations, which lead to joints with a minimum thickness of, for instance, 30 μm between the ceramic parts.
The desire for miniaturization of pressure sensors means indirectly a need for a thinner joint. For example, in the case of a ceramic pressure sensor with a capacitive transducer, miniaturization leads to a reduction of the electrode areas of the capacitive transducer, which then must be compensated by a lessening of the distance.